The present invention relates to hydroxide-coated rare earth oxide particles or, more particularly, to rare earth oxide particles having a coating layer of the rare earth hydroxide having a specific thickness and useful, particularly in the form of an aqueous slurry, as the base material of a sintered body of the rare earth oxide and as a sintering aid in the manufacture of sintered ceramic bodies of various kinds of inorganic materials such as alumina, zirconia, sialon, silicon nitride, aluminum nitride and boron nitride to serve as a structural or functional element in fine technologies such as electronics.
Along with the progress in the technology of the above mentioned ceramic materials, it is a remarkable trend in recent years that use of powders of a rare earth oxide is increasing year by year as a base material of sintered ceramic body of the rare earth oxide per se or as a sintering aid in the manufacture of other ceramic materials. In these applications, the rare earth oxide powder is used in most cases in the form of a slurry prepared by dispersing the rare earth oxide powder in a suitable liquid medium using a dispersing or mixing machine such as ballmills and media mills. The liquid as the above mentioned dispersion medium can be an organic solvent or can be water but water is used increasingly as the dispersion medium in place of organic solvents in consideration of the problems relating to safety against fire and workers' health as well as environmental pollution.
One of the problems encountered in the use of a rare earth oxide powder in the form of an aqueous slurry or slip with water as the dispersion medium is stability of the rheological properties of the slurry in storage relative to the viscosity or consistency thereof. Namely, the viscosity of an aqueous slurry of a ceramic powder is a factor having great influences on the workability in the cast molding process affecting the quality such as dimensional accuracy of the molded green bodies and yield of acceptable molded bodies. The rheological properties of an aqueous slurry of a ceramic powder are also greatly influenced by the types and amounts of the dispersing agent and binder admixed to the slurry together with the base powder of the ceramic material so that the formulation of aqueous slurries must be modified depending on the molding conditions of the slurry into shaped green bodies as well as the types of the final ceramic products. In view of the large changes caused in the properties of the aqueous slurry during storage, it is not practical to prepare a large volume of an aqueous slurry of a ceramic powder at one time for storage but must be prepared in small volumes before each run of the molding process thus to greatly decrease the productivity of the ceramic molding process.